It's not every day that you get to see evolutionary pressures at work, but that's exactly what Sharon Jansa (U. of Minnesota) and Robert Voss (American Museum of Natural History) found when they started studying (of all things) the Virginia opossum. In looking at opossums' genetic data, they found an unusually high amount of differences in genes coding for a blood protein called "von Willebrand's Factor" (vWF). This blood protein is a target of snake venom, which usually attacks vWF to cause the massive internal bleeding that leads to venom-induced death in snakebite victims.
Why would opossums have such a huge amount of differences in their vWF gene? The reason lies in the evolutionary battle that's constantly being waged by the opossum and the venomous snakes that they eat. Snake venom proteins also have an incredibly high amount of variation in their genes. This, in turn, encourages variation in the opossum defenses against that venom (their vWF).
It's a classic example of evolutionary pressure, where there's a clear advantage to having a variety of different genes in the gene pool (variety = fewer opossums dying from eating the local snakes!) It's not often that there is that strong an evolutionary pressure (instant death for having the wrong gene will do that!), so seeing a relationship like this between predator and prey is kind of awesome.
Jansa SA, Voss RS (2011) Adaptive Evolution of the Venom-Targeted vWF Protein in Opossums that Eat Pitvipers. PLoS ONE 6(6): e20997. doi:10.1371/journal.pone.0020997